Make the flame of a candle more yellow
A few options:

Using the compositor, changing the materials for the emissions shader, or harnessing the power of color magangement.

Each one will have a different effect, and can also be combined according to your artistic intentions.

Option 1: Using the compositor you want change the color for the flame so it doesn't look white.

(click on any of the images to enlarge)



To isolate the flame from the rest of the image you can enable the emissions pass on the render layers.



On the compositor you can see that you have a new image that contains only the emissions of the image.



You can then use this information to tint the image, so that only the flame is affected.

Plug the Emit information to a vector normalize node (to bring the values to a scale form 0 to 1), and plug the result to a color mix node's factor. That will allow you set the color you want the flame to be. Ony the flame will be affected by the color selected on the Mix node. 



Note that the edges are a bit rough since the emissions pass is not antialiased, so you can add a Dilate/Erode node in Feather node to make the edges smoother:



Adding a blur filter to the emissions pass will allow you to create some glow...



Here's the blend file if you want to download it:



Now in my opinion that still looks like a colored light bulb instead of a flame, so we can try now

Option 2. Using more than one emitter shader for the flame.

The flame material for a candle is hardly ever just a single color emitter. 

Here is a different example file to play with:



You can start with a node setup like this for the material of the flame:


(Click on the image to enlarge)

It can be broken in 3 main components:


First, a gradient for the different temperatures of the flame.
A first emitter that is mixed with some transparency. This is the one that will determine how the flame will look in terms of color, brightness and transparency.


The biggest issue is that with just one emitter is that if you make the flame brighter to illuminate surrounding objects it just becomes white, so you need to add


a second emitter that is unseen by the camera, but that illuminates everything else (The light path node prevents the second emitter to be seeing by the camera).


If you look at this example, this second emitter is 100 times brighter than the flame.



Then you can add more effects on the compositor:



Option 3: Using Tone Mapping in the compositor

With a single emission shader, using a gradient to create a more realistic flame.

Again, in order to see the color on the flame, the emission strength would need to be low, but that makes the surroundings very dark.



Bringing up the emission's strength will make the scene look brighter but the flame's gradient is way to bright now, past the 100% clipping point



Looking at the waveform monitor we can see that the flame is way over the 100% mark, so all detail and color is gone, no more information can be displayed on white.



Using a Tonemap node we can try to modify the dynamic range of the scene by remapping of the values to fit on a scale of 0(black) to 1(white):



The colors for the flame are back, but the image looks very washed out and boring because it has a much lower contrast and less color saturation. To fix that we can alter the image values with a color curve.



Lastly just add a glare node:





Option 4 Using color management (as suggested by this answer)

Using a custom 3dLUT (Look Up Table) for the device transform you can use a much wider dynamic range on the scene.



Again, with a single emitter shader on the flame, a LUT used on the color management will allow us to use luminance values much brighter than 1 and re-map them to fit a range between 0 and 1.

For more information read this post.

User @troy_s has created a set of LUTs and OCIO config that greatly enhances blender's cycles rendering and its ability to deal with a wider dynamic range, thus creating more photorealistic results. They can be downloaded here, along with the instructions on how to install and use.